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Ultrahigh Thermal Conductivity in Isotope-Enriched Cubic Boron Nitride

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Ultrahigh Thermal Conductivity in Isotope-Enriched Cubic Boron Nitride Ultrahigh thermal conductivity in isotope-enriched cubic boron nitride The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Chen, Ke et al. "Ultrahigh thermal conductivity in isotope-enriched cubic boron nitride." Science 367, 6477 (January 2020): 555-559 © 2020 The Authors As Published http://dx.doi.org/10.1126/science.aaz6149 Publisher American Association for the Advancement of Science (AAAS) Version Author's final manuscript Citable link https://hdl.handle.net/1721.1/127819 Terms of Use Creative Commons Attribution-Noncommercial-Share Alike Detailed Terms http://creativecommons.org/licenses/by-nc-sa/4.0/ Ultrahigh thermal conductivity in isotope-enriched cubic boron nitride Ke Chen1*, Bai Song1*†‡,, Navaneetha K. Ravichandran2*, Qiye Zheng3§, Xi Chen4#, Hwijong Lee4, Haoran Sun5, Sheng Li6, Geethal Amila Gamage5, Fei Tian5, Zhiwei Ding1, Qichen Song1, Akash Rai3, Hanlin Wu6, Pawan Koirala6, Aaron J. Schmidt1, Kenji Watanabe7, Bing Lv6, Zhifeng Ren5, Li Shi4,8, David G. Cahill3, Takashi Taniguchi7, David Broido2†, and Gang Chen1† 1Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 2Department of Physics, Boston College, Chestnut Hill, MA 02467, USA. 3Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA. 4Materials Science and Engineering Program, Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, USA 5Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA. 6Department of Physics, The University of Texas at Dallas, Richardson, TX 75080, USA. 7National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan. 8Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA. ‡Current address: Department of Energy and Resources Engineering, and Beijing Innovation Center for Engineering Science and Advanced Technology, Peking University, Beijing 100871, China §Current address: Lawrence Berkeley National Laboratory, and Department of Mechanical Engineering, University of California, Berkeley, CA 94720 #Current address: Department of Electrical and Computer Engineering, University of California, Riverside, CA 92521, USA †Corresponding author. Email: [email protected] (B.S.); [email protected] (D.B.); [email protected] (G.C.) *These authors contributed equally to this work. 1 Abstract: Materials with high thermal conductivity (k) are of technological importance and fundamental interest. We grew cubic boron nitride (cBN) crystals with controlled abundance of boron isotopes and measured k over 1600 Wm-1K-1 at room temperature in samples with enriched 10B or 11B. In comparison, we found the isotope enhancement of k is considerably lower for boron phosphide and boron arsenide as the identical isotopic mass disorder becomes increasingly invisible to phonons. The ultrahigh k in conjunction with its wide bandgap (6.2 eV) makes cBN a promising material for microelectronics thermal management, high-power electronics, and optoelectronics applications. One Sentence Summary: A high thermal conductivity was found in isotopically enriched cBN. Main Text: Ultrahigh thermal conductivity (k) materials are desirable for thermal management, and have long been a subject of both fundamental and applied interest (1). Despite decades of effort, only a few materials are known to have an ultrahigh thermal conductivity, which we define as exceeding 1000 Wm-1K-1 at room temperature (RT) (2). In metals, free electrons conduct both charge and heat. Therefore, the best electrical conductors like silver and copper also have the highest k for metals. In semiconductors and insulators, phonons carry the heat. The intricate interplay between lattice dynamics, anharmonicity, and defects dictates thermal transport. Despite the large number of materials that have phonon-dominated heat transport, since 1953 diamond is recognized as the most thermally conductive bulk material at RT (3). Besides diamond, a set of non-metallic crystals with high k was systematically identified by Slack in 2 1973 (4), including silicon carbide (SiC), boron phosphide (BP), and the cubic, zincblende polymorph of boron nitride (cBN) (5). In addition, Slack (4) proposed guidelines for searching for crystals with high k, suggesting candidates should be composed of strongly-bonded light element(s) arranged in a simple lattice with low anharmonicity. These guidelines were established via approximate models but captured the essential need for high phonon group velocity and low phonon scattering rates. Since Slack’s work, the k of diamond (~2000 Wm-1K-1 with natural carbon isotopes at RT) (6, 7) has not been surpassed among bulk materials. High thermal conductivities of up to 490 Wm-1K-1 and 768 Wm-1K-1 were reported for BP and cBN (8–12), respectively, benefiting from progress in crystal growth and thermal characterization techniques. Unlike cBN and BP, boron arsenide (BAs) has the much heavier arsenic element and was originally estimated to have -1 -1 a kRT of 200 Wm K (4). In 2013, Lindsay, Broido and Reinecke (13, 14) showed with ab initio simulations that BAs should have a kRT rivaling that of diamond due to a dramatic reduction in the strength of the lowest-order processes giving intrinsic thermal resistance, three-phonon -1 -1 scattering. Several experiments demonstrated in 2018 a kRT of ~1200 Wm K (11, 15, 16), making BAs one of the most thermally conductive materials, and consistent with modified predictions that included four-phonon scattering (16, 17). -1 -1 Apart from the unusual BAs, cBN was predicted to have a kRT exceeding 2000 Wm K upon isotopic enrichment of the boron atoms using theories that ignored four-phonon scattering (13, 18, 19). We combined experimental characterizations with ab initio simulations that include four-phonon scattering to revisit heat transport in cBN, using synthetic crystals with natural (natB) and controlled abundance of boron isotopes. We demonstrated experimentally that cnatBN 3 -1 -1 10 11 - crystals can have a kRT over 850 Wm K and enriched c B (or B) N can reach over 1600 Wm 1K-1. The ultrahigh k we measured was consistent with our first-principles calculations, which showed relatively weak effects of higher order anharmonic phonon-phonon interactions on k in cBN. Furthermore, the ~90% enhancement of kRT upon boron isotope enrichment qualitatively supported prior calculations (13, 18, 19) and represents a very large RT isotope effect. For isotope-controlled BP and BAs, we only calculated a 31% and a 12% increase in kRT, which agreed with the small isotope effect we measured. We used simulations to discover the differences between these boron pnictides which can only be understood by considering the subtle interplay between the mutual interactions involving phonons and isotopic disorder. We prepared four sets of cBN crystals combining natural nitrogen (99.6% 14N and 0.4% 15N) with different boron isotope compositions including natB (21.7% 10B and 78.3% 11B), enriched (99.3%) 10B, enriched (99.2%) 11B, and a roughly equal mix of 10B and 11B (eqB, 53.1% 10B and 46.9% 11B). The cnatBN crystals were obtained by a conventional process using commercial hexagonal boron nitride (hBN, with natB) crystals as a starting material (20–22). Because no boron isotope-controlled hBN crystals were commercially available, we grew the m other cBN crystals with a metathesis reaction of Na BH4 + NH4Cl under high pressure, where m 10 11 10 11 B is B or B (22). By controlling the mixing ratio of Na BH4 and Na BH4, we achieved the desired boron isotope ratios. We obtained nearly colorless cnatBN crystals with the conventional process, while the isotope-controlled cBN crystals from the metathesis reaction generally were a light amber color (Fig. 1A and fig. S1). We made single-crystal X-ray diffraction measurements on a c10BN sample (XRD, Fig. 1B and fig. S1) and found a cubic structure with the �4´ 3� space group with a refined lattice constant of 3.6165(5) Å, in good agreement with literature values (5). Peaks corresponding to different crystallographic directions were observed, indicating the 4 presence of multiple crystallites (22). We measured the isotope compositions of the cBN crystals using time-of-flight secondary ion mass spectrometry (TOF-SIMS) (Fig. 1C). We also characterized impurities using TOF-SIMS (fig. S2), and found they mainly consisted of carbon and oxygen, consistent with previous results (21). In addition, we used Raman spectroscopy to identify the isotope compositions (Fig. 1D). As the average mass of boron increases from c10BN to c11BN, the characteristic Raman peaks for the optical phonons at the Brillouin zone-center red-shift noticeably, scaling with the square root of the reciprocal reduced mass and in good agreement with simulations (fig. S27 and Table S1). We subsequently selected cBN crystals with flat facets of ～200 µm lateral dimension (Fig. 1A) for thermal transport measurements using the laser pump-probe techniques of time- and frequency-domain thermoreflectance (TDTR and FDTR, respectively, Fig. 2, figs. S3-19, eq -1 -1 and Table S4) (11, 15, 16, 22). The c BN crystals yielded the lowest κRT of 810 ± 90 Wm K 10 11 -1 -1 nat with 53.1% B and 46.9% B. A higher κRT of 880 ± 90 Wm K was found for the c BN crystals with 78.3% 11B, which was also higher than previously reported measurements (Fig. 3C) 11 - (11, 12). As we further enriched the B isotope to 99.2%, we observed a κRT of 1660 ± 170 Wm 1 -1 11 -1 -1 K in the c BN crystals. Likewise, we measured an ultrahigh κRT of 1650 ± 160 Wm K in the c10BN crystals with 99.3% 10B, which we confirmed using a different TDTR platform that gave a -1 -1 κRT of 1600 ± 170 Wm K (Fig. 3, inset). We found no substantial effect from the metal transducer layer by using both gold and aluminum (Fig.
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